1. Field of the Invention
This invention relates to a dredging apparatus for extracting bottom-dwelling shellfish such as clams, oysters, mussels, cockles, and crabs from a sea, lake, or riverbed, and for continuously transporting the extracted shellfish to the surface.
More specifically, the invention relates to a hydraulic dredging apparatus that includes a source of pressurized water, at least one water jet arranged to receive water from the pressurized water source and direct it at shellfish-containing sediments, sorting plates for receiving the shellfish-containing sediments excavated by the water jet or jets and separating the shellfish from the sediments, a collection chamber for receiving the separated shellfish, and dual lifting compartments, one of which is connected to the pressurized water source for lifting shellfish from the collection chamber and entraining the shellfish for transport to the surface, and the other of which is arranged to received pressurized air for increasing the transport speed and lifting power while cushioning the shellfish as they are transported to the surface. Unlike previous hydraulic transport mechanisms, the dual-compartment air/water transport arrangement of the invention permits precise control of excavation and transport pressure, without the need for multiple air or water sources and/or manifolds.
2. Description of Related Art
Numerous attempts have been made to devise mechanical harvesters that move or that can be towed along the bottom of a body of water in order to harvest shellfish that live in colonies at the bottom. All of these devices seek to dredge shellfish such as clams, oysters, cockles, mussels, and/or crabs from the bottom of the body of water and either trap the shellfish for retrieval after the device is brought to the surface, or continuously transport the shellfish to the surface as the dredge is being towed along the bottom.
Common problems that the designers of these devices have attempted to solve include problems of efficiency, i.e., the relationship between power or effort expended and the amount of shellfish harvested, problems related to the cost and reliability of the device, which are often a function of complexity, and problems related to environmental damage caused by the device as it is towed across the sea, lake, or riverbed.
The earliest attempts at large-scale shellfish harvesting devices undoubtedly date back to prehistoric times and most likely involved diggers or tongs dragged along the bottom for scooping shellfish into a collection cage or basket that could then be brought to the surface and emptied. More sophisticated but nevertheless fundamentally similar examples of dredges of this type are still being used and are disclosed, for example, in U.S. Pat. Nos. 4,827,635, 4,425,723, and 3,226,854. Such dredges have the advantage of simplicity, but are relatively inefficient because of inherent limitations in the effectiveness of mechanical dredging devices, and the need to repeatedly bring the dredges to the surface to be emptied.
As early as Greek times, high pressure jets of water capable of slicing horizontal layers of sediment were being used to hydraulically harvest shellfish, the loosened or liquified mass of shellfish-containing sediments being sifted to separate the shellfish from the sediments, and the remaining shellfish being collected in a collection cage or basket before being brought to the surface. Such harvesters, which are also still in use, have the advantage of being able to dredge a relatively large area in less time than a purely mechanical harvester, although they still require the collection cage or basket to be periodically brought to the surface for emptying.
In order to avoid the need to periodically bring the dredge to the surface for removal of harvested shellfish, numerous generally unsuccessful attempts have been made to add conveyors that continuously and automatically convey recovered shellfish to the surface, either in connection with a purely mechanical harvester, or in connection with a harvester that uses a hydraulic digging action. These conveyors initially involved purely mechanical conveyance systems in the form of conveyor belts or escalators, but were limited to use in relatively shallow waters since systems of greater length involved intractable problems in handling and complexity. A recent example of a non-hydraulic dredge with a mechanical conveyor is disclosed in U.S. Pat. No. 4,464,851, while examples of hydraulic dredges with mechanical conveyors are disclosed in U.S. Pat. Nos. 2,508,087, 3,462,858, and 3,521,386.
In theory, hydraulic means of conveying extracted shellfish to the surface through pipes or hoses appear to offer greater simplicity and ease-of-handling than purely mechanical conveyance systems, and therefore the possibility of use at greater depths. However, in practice, most of the previously proposed hydraulic conveyance systems have suffered from slow speed, excess energy consumption and, in the case of systems that share water jets for both excavation and transport, difficulties in controlling excavation and transport pressures. For example, the system disclosed in U.S. Pat. No. 3,624,932 requires separate pumps, two corresponding pressurized water lines, and a transport hose to carry out excavation and transport of shellfish to the surface, resulting in relatively high power consumption and an increased possibility of tangling or breakage. The system disclosed in U.S. Pat. No. 3,184,866 utilizes both air and water for excavation as well as transport, and therefore requires even more hydraulic lines including, as illustrated in FIG. 1 appended hereto, two pressured water lines 49 and 50 with corresponding manifolds 43 and 46, two pressurized air lines 48 and 41, and a transport hose 20 to carry the excavated shellfish to the surface. Possibly because of the number of lines required, the system of U.S. Pat. No. 3,184,866 requires both a tow boat and a receiving boat or installation.
More recently, it has been proposed to use the same source of pressurized water for both the excavating jets and transport system of a hydraulic dredging apparatus, thereby eliminating the need for separate hydraulic lines and/or sources. The decrease in water pressure available for transport is compensated for by an improved transport system in which collected shellfish are siphoned rather than pushed or swept out of the collection chamber. Examples of systems in which jets of water are used to hydraulically separate shellfish from sediments, and also to create a siphon or Venturi effect that lifts the separated shellfish into a stream of water and carries them to the surface, are disclosed in British Patent Publication GB1,156,547 and U.S. Pat. No. 6,237,259. The latter system is illustrated in FIGS. 2 and 3, appended hereto.
In the system disclosed in U.S. Pat. No. 6,237,259, which was developed by the present inventor, the dredging apparatus includes a sled having a main frame 40 and a digging blade 21 that is inclined forwardly and downwardly relative to the frame so as to extend below the bottom of the frame into the sediments to be dredged. A digging jet pipe 22 is fixed relative to the front surface of the digging blade 21 and is arranged to discharge water under pressure on to the surface of the seabed immediately ahead of the digging blade to fluidize the sediments as they pass onto the blade. The angle of the digging blade 21 is such that a surface section of the seabed cut by the blade travels up the slope of the blade and into the open end or mouth 23 of the frame 40. Water to the digging jet 22 is supplied by a pump situated on a vessel through a hose 24 connected by suitable fittings to the digging jet. Extending rearwardly from digging blade 21 is a first separating device 25 made up of a plurality of horizontal bars 26,27,28 arranged in a direction generally parallel to a direction of travel of the apparatus as it is towed by a vessel, for separating shellfish collected by the digging blade from sediments in which the shellfish are entrained, and also for separating out immature shellfish having a size smaller than that of the shellfish to be collected. To the rear of the first separating device 25 is a second separating device 29 in the form of a plate 30 having a plurality of openings 31 arranged to permit passage of shellfish while excluding larger objects, including clumps of sediment not completely liquified by the digging water jet.
The hydraulic transport system of the apparatus illustrated in FIGS. 1 and 2 includes a plate 30 that forms the top of a suction chamber 32 at the rear of the sled, and includes an opening 33 having a larger diameter than any of openings 31. Opening 33 is provided with a fitting for attachment of a transport tube 34 extending to the towing vessel. Transport tube 34 is connected by a hose or pipe 35 to the hose 24 that also supplies water to the digging jet. Nozzles 36 serve to direct pressurized water from hose or pipe 35 towards the surface in the direction of conveyance. The stream of water from the nozzles creates a siphon effect in the direction of arrow B to draw shellfish present in the suction chamber into the conveyance tube for transport to the towing vessel. A reduced diameter portion 37 of tube 34 situated immediately below the nozzles 36 increases the velocity of water being drawn past the nozzles so as to decrease the pressure in tube 34 in the area above suction chamber 32 and thereby increase the suction force and the efficiency by which shellfish in the suction chamber are transported to the surface.
While more energy efficient, versatile, easy-to-handle, and reliable than prior dredging apparatuses, the dredging apparatus illustrated in FIGS. 2 and 3 has the disadvantage that, in order to control the suction and lifting power of the transport conveyor to enable use of the conveyor at greater depths and in a wider variety of marine environments, it is necessary to increase or decrease the pressure supplied to the digging jet 22. The resulting variations in digging or excavation pressure make it difficult to control extraction, and may lead to excess energy use, undue disturbance of the bottom, and possibly damage to extracted shellfish or other marine organisms.
It is accordingly a first objective of the invention to provide a relatively low cost, high performance arrangement for harvesting shellfish from the bottom of a body of water, and for continuously conveying the harvested shellfish to a boat.
It is a second objective of the invention to provide an arrangement for harvesting shellfish from the bottom of a body of water in which conveyance of harvested shellfish to the surface is carried out primarily by the same source of hydraulic pressure that is used to extract shellfish from sediments, and yet that includes a secondary source of hydraulic pressure independent of the excavation pressure source that can be used to increase or control the transport pressure without varying the excavation pressure, thereby minimizing disturbance of the bottom and damage to the beds from which the shellfish are extracted, and/or to the shellfish being extracted, while permitting the apparatus to be used at arbitrary depths.
It is a third objective of the invention to provide an arrangement for conveying shellfish from a dredge to the surface at increased speeds and with minimal damage to the shellfish being conveyed.
It is a fourth objective of the invention to increase the lifting pressure of a hydraulic conveyor so as to minimize clogging or blockage.
It is a fifth objecting of the invention to provide a combined hydraulic excavation and transport system that offers a common source of excavation pressure and transport pressure, efficient suction-based lifting of excavated materials into the transport stream, and separate control of excavation and transport pressures.
These objectives are achieved by providing a shellfish harvesting apparatus in the form of a sled towed and equipped with hydraulic lines that direct pressurized water rearwardly relative to the direction of travel of the sled. The pressurized water sweeps sediments and shellfish towards a separator device that separates the shellfish from the sediments, after which the pressurized water sweeps the separated shellfish towards a suction chamber where the pressurized water creates a Venturi effect, causing shellfish entering the chamber to be transported to the surface through a trunk line.
According to the principles of a preferred embodiment of the invention, the suction chamber includes dual lifting compartments. The first compartment is situated above a collection chamber in the dredge and is arranged to receive a stream of pressurized water, which creates a Venturi effect that lifts shellfish from the collection chamber into the stream for transport through a hose to the surface. The second compartment is situated above the first compartment and is arranged to receive one or more air jets for adding velocity and lifting power to the transport stream, and for cushioning the shellfish as they make their way up the hose to the surface.
As a result, the present invention combines the efficiency and ease-of-handling of a siphon-based hydraulic dredging/transport system of the type disclosed in U.S. Pat. No. 6,237,259, with the enhanced excavation and transport pressure control potentially offered by a systems having separate excavation and transport lines, such as the one disclosed in U.S. Pat. No. 3,184,866.